Incremental optical encoders are frequently used for resolving the position or measuring the rotation of a shaft. For 15 example, optical encoders are widely used in instruments such as plotters to monitor the speed and shaft position of internal electric motors. Optical encoders are also used in devices for controlling the output of electronic instruments. In instruments such as oscilloscopes, for example, the resolution or frequency of the oscilloscope display is often adjusted by means of an optical encoder coupled to a control knob or wheel.
In typical prior art applications, as shown in FIG. 1, an opaque code wheel 100 is coupled to the shaft 110 of a control knob 111 so that movement of the shaft results in like movement of the code wheel. Wheel 100 contains a plurality of transparent windows 101 in a track around the circumference of the wheel. A light source 102 is provided adjacent to one face of wheel 100 and in alignment with the track of windows 101. A photo detector on printed circuit board 103 is placed on the side of wheel 100 opposite light source 102. As wheel 100 rotates, illumination from light source 102 passes through windows 101 and is detected by the photo detector. Electronic circuitry connected to the photo detector is used to determine the number of windows 101 which pass light source 102 as wheel 100 rotates. A phase plate 104 is mounted to the encoder housing 105 and aligned concentrically relative to shaft 110 such that the relative angle between phase plate 104 and code wheel 100 changes as the shaft rotates.
According to the operation of the encoder shown in FIG. 1, a modulated light beam passing through the track of windows is received by the areas 106 and 107 of phase plate 104. Phase plate areas 106 and 107 are oriented out of phase relative to the periodic spacing between windows 101 of code wheel 100. Thus, as code wheel 100 turns, the light passing through phase plate areas 106 and 107 alternately achieves a maximum and a minimum. That is, when the light passing through optical area 106 reaches a maximum, the light passing through optical area 107 reaches a minimum. The light passing through area 106 is received by a first photo detector and the light passing through are 107 is received by a second photo detector. The photo detectors are connected to electronic circuitry which generates an output signal indicative of the number of times the light intensity from each area is of equal intensity. Such a signal is indicative of the relative angle between code wheel 100 and phase plate 104.
The code wheel and phase plate shown in FIG. 1 utilize trapezoidal shaped light transmissive areas. Prior art encoders of the type disclosed in FIG. 1 have also employed code wheels having spiral-shaped transmissive areas. In such devices, the phase plate is configured according to a series of complex, iterative steps so as to ensure proper modulation of the light beams from light source 102. Separate photo detectors are also used in such devices.
A recent advance in the art relates to elimination of the phase plate from optical encoder configurations. Such encoders, as disclosed in U.S. Pat. No. 4,691,101-Leonard, utilize interdigitated light detector arrays to perform the function previously performed by the phase plate. U.S. Pat. No. 4,691,101-Leonard (Leonard) is incorporated herein in its entirety. The photo detector arrays comprise, for example, four separate photo detectors which are immediately adjacent to one another and which receive information belonging to two channels. The array produces four output signals AB, AB, AB and AB, one for each individual photo detector. These four signals are preferably produced in quadrature, that is, they are identically shaped and are 90.degree. out-of-phase with each other.
While the arrangement disclosed in Leonard eliminates the need for a phase plate, it imposes certain other restrictions on the encoder configuration. For example, Leonard teaches that the photo detector array must have the same size and shape as one transmissive section and one non-transmissive section of the code wheel. Thus, Leonard discloses a code wheel with trapezoidal windows and a substantially identically sized and shaped photo detector array.
While the encoders taught by Leonard represent a significant advance in the art, several disadvantages are associated with the arrangement disclosed therein, especially when relatively low cost encoders are desired. As explained in co-pending application Serial No. 321,994, one disadvantage stems from having the light source and the light detectors on opposite sides of the wheel. This requires the presence of two substrates to which electrical connections must be made, which in turn complicates the positioning and alignment of components to obtain a desired precision. Each of these adds significant cost to the encoder.
Another difficulty associated with the Leonard encoder arises in applications requiring low count-per-revolution (CPR) encoders. As described by Leonard, the CPR of an optical encoder is the number of cycles the four output signals AB, AB, AB and AB go through in one revolution of the code wheel. As is understood by those skilled in the art, many optical encoder applications require low CPR code wheels. For example, it is desirable to utilize low CPR code wheels in connection with fine tuning control of electronic equipment. It is equally desirable for encoders and the components associated therewith to be of the smallest possible size Applicant has found, however, that it is not generally possible to simultaneously achieves both of these objectives using encoders with code wheels as taught by Leonard.
In the configuration of Leonard each photo detector array must have the same dimensions as one transmissive section plus one non-transmissive section of the code wheel. Moreover Leonard teaches that the transmissive and non-transmissive areas of the code wheel are of the same size and shape. According to the teachings of Leonard, therefore, the width of the photo detector array must increase in order to accommodate low CPR code wheels. This is contrary to the objectives of low cost and small size.